U.S. patent number 7,769,466 [Application Number 11/678,263] was granted by the patent office on 2010-08-03 for class-e radio frequency amplifier for use with an implantable medical device.
This patent grant is currently assigned to Kenergy, Inc.. Invention is credited to Arthur J. Beutler, Cherik Bulkes, Stephen Denker.
United States Patent |
7,769,466 |
Denker , et al. |
August 3, 2010 |
Class-E radio frequency amplifier for use with an implantable
medical device
Abstract
A medical apparatus includes an extracorporeal power source that
transmits electrical power via a radio frequency signal to a
medical device implanted inside an animal. The extracorporeal power
source has a Class-E amplifier with a choke and a semiconductor
switch connected in series between a source of a supply voltage and
circuit ground. An output node of the amplifier is formed between
choke and the switch and connected to a transmitter antenna. A
shunt capacitor couples the amplifier's output node to the circuit
ground. Controlled operation of the switch produces bursts of the
radio frequency signal that are pulse width modulated to control
the amount of energy being sent to the implanted medical
device.
Inventors: |
Denker; Stephen (Mequon,
WI), Bulkes; Cherik (Sussex, WI), Beutler; Arthur J.
(Greendale, WI) |
Assignee: |
Kenergy, Inc. (Mequon,
WI)
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Family
ID: |
38255826 |
Appl.
No.: |
11/678,263 |
Filed: |
February 23, 2007 |
Prior Publication Data
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Document
Identifier |
Publication Date |
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US 20070210862 A1 |
Sep 13, 2007 |
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Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
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60776853 |
Feb 24, 2006 |
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Current U.S.
Class: |
607/61; 607/32;
607/33 |
Current CPC
Class: |
H03F
1/34 (20130101); A61N 1/3787 (20130101); H03F
3/217 (20130101); H03F 3/2176 (20130101); H03F
2200/351 (20130101) |
Current International
Class: |
A61N
1/00 (20060101) |
Field of
Search: |
;607/61,32,60,33 ;330/10
;128/903 ;361/8 ;340/539 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
Primary Examiner: Bockelman; Mark W
Assistant Examiner: So; Elizabeth K
Attorney, Agent or Firm: Haas; George E. Quarles & Brady
LLP
Parent Case Text
CROSS-REFERENCE TO RELATED APPLICATIONS
This application claims benefit of U.S. Provisional Patent
Application No. 60/776,853 filed Feb. 24, 2006.
Claims
What is claimed is:
1. A medical apparatus comprising: an extracorporeal power source
having a power transmitter that produces a radio frequency signal
and an antenna that is connected to the power transmitter, the
power transmitter including a source of the radio frequency signal,
a Class-E amplifier that comprises a switch; the power source
comprising a pulse width modulator that receives the radio
frequency signal and a control signal and in response thereto
produces an output signal that is applied to a control input of the
switch; an output sensor which provides a feedback signal
indicating an intensity of radio frequency signal applied to the
antenna, and wherein the pulse width modulator responds to the
feedback signal by varying the output signal; and a medical device
for implantation inside an animal, the medical device receiving the
radio frequency signal transmitted from the antenna and deriving an
electrical voltage from energy of the radio frequency signal which
electrical voltage powers components of the medical device.
2. The medical apparatus as recited in claim 1 wherein the Class-E
amplifier further comprises a choke connected in series with the
switch between a source of a supply voltage and circuit ground,
with an amplifier output node being formed between the choke and
the switch and connected to the antenna, and a shunt capacitor
coupling the amplifier output node to the circuit ground.
3. The medical apparatus as recited in claim 1 wherein the switch
is selected from a group consisting of a semiconductor device and a
MOSFET.
4. The medical apparatus as recited in claim 2 wherein the switch
is a semiconductor device that is connected to a load and the radio
frequency signal is proportional to an induced voltage at the
load.
5. The medical apparatus as recited in claim 1 wherein the output
signal controls a duty cycle of the switch.
6. The medical apparatus as recited in claim 5 wherein the switch
is a MOSFET that has a channel resistance and a peak current
rating, wherein an arithmetic product of the channel resistance and
a peak current rating is less than 3% of a supply voltage to the
Class-E amplifier.
7. The medical apparatus as recited in claim 1 wherein the switch
is rated to conduct a transient current level that is at least ten
times a maximum level of a current that the switch is expected to
conduct.
8. The medical apparatus as recited in claim 1 wherein the pulse
width modulator modulates the radio frequency signal with the
control signal to produce the output signal.
9. The medical apparatus as recited in claim 1 wherein the output
signal has an envelope defined by the control signal.
10. The medical apparatus as recited in claim 1 wherein the output
signal comprises pulses of the radio frequency signal, wherein each
pulse has a shape that is defined by the control signal.
Description
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT
Not Applicable
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to implantable medical devices, which
deliver energy to stimulate tissue for the purposes of providing
therapy to the tissue of an animal, and more particularly to an
radio frequency amplifier for use in such a medical device.
2. Description of the Related Art
A remedy for a patient with a physiological ailment is to implant
an electrical stimulation device that provides provide therapy to
the patient. An electrical stimulation device is a small electronic
apparatus that stimulates an organ or part of an organ with
electrical pulses. It includes a pulse generator, implanted in the
patient, and from which electrical leads extend to electrodes
placed adjacent to specific regions of the organ.
An improved apparatus for physiological stimulation of a tissue
includes a wireless radio frequency (RF) receiver implanted as part
of a transvascular platform that comprises at least one
stent-electrode that is connected to the wireless RF receiver and
an electronic capsule containing a stimulation circuitry. The
stimulation circuitry receives the radio frequency signal and, from
the energy of that signal, derives an electrical voltage for
powering the implanted device. The electrical voltage is applied in
the form of suitable waveforms to the electrodes, thereby
stimulating the tissue of the organ.
The radio frequency (RF) signal generation is a significant part of
the electrical stimulation apparatus and it usually involves the
use of an RF amplifier. The RF amplifier of choice typically has
been a Class-A or Class-AB amplifier in those cases where linearity
is of utmost concern. The class of an analog amplifier defines what
proportion of the input signal cycle is used to actually switch on
the amplifying device. A Class-A amplifier is switched on 100% of
the time. A Class-AB amplifier uses a signal cycle that is greater
than 50%, but less than 100% to switch on the amplifying device.
Unfortunately, these amplifiers are not very efficient and
dissipate a significant amount of energy. The efficiency of a power
amplifier is defined as the ratio of output power and input power
expressed as a percentage.
Recently, a different kind of amplifier, known as a switching
amplifier, has been developed. A particularly useful switching
amplifier is called a Class-E amplifier. Switching amplifiers have
relatively high power efficiency due to the fact that perfect
switching operation does not dissipate power. An ideal switch has
zero impedance when closed and infinite impedance when open,
implying that there is zero voltage across the switch when it
conducts current (on state) and zero a non-zero voltage across it
in the non-conductive state (off state). Consequently, the product
of voltage and current (power loss) is zero at any time. Therefore,
a Class-E amplifier has a theoretical efficiency of 100%, assuming
ideal switching.
From a theoretical standpoint, a Class-E amplifier can provide very
efficient RF amplification. However, in practice, Class-E
amplifiers do not achieve anywhere close to the theoretical limits.
Some embodiments of the prior art techniques use a relaxation
oscillator to drive the amplifier. With this technique, it is
impossible to control the range of the power depending on the need.
In other embodiments, a regulator is used to control the power
feed. In this case, heat is generated in the control system itself
and the amplifier's efficiency is subsequently lowered. Therefore,
there is a need to improve the performance of practical Class-E RF
power amplifiers based on the fundamental understanding of the loss
generation processes. An optimal design can make the heat
dissipation so low such that heat-sink are not required.
SUMMARY OF THE INVENTION
The present invention provides a Class-E RF power amplifier
suitable to be used in an extracorporeal power source that supplies
a medical device implanted inside an animal.
The extracorporeal power source comprises a power transmitter that
produces a radio frequency signal and an antenna that is connected
to the power transmitter. The power transmitter includes a Class-E
amplifier that has a choke and a switch connected in series between
a source of a supply voltage and circuit ground. An amplifier
output node is formed between choke and the switch and is connected
to the antenna. A shunt capacitor couples the amplifier output node
to the circuit ground.
The medical device receives the radio frequency signal transmitted
from the antenna and derives an electrical voltage from energy of
the radio frequency signal in order to power components of the
medical device.
In a preferred embodiment, the switch comprises a semiconductor
device, such as a MOSFET. Ideally the semiconductor device has a
feedback capacitance that is less than 10% of its input
capacitance. It also is preferred that channel resistance and a
peak current rating of the semiconductor device are such that the
arithmetic product of the channel resistance and the peak current
rating is less than 3% of the supply voltage.
BRIEF DESCRIPTION OF DRAWINGS
FIG. 1 is a schematic diagram of a wireless transvascular platform,
that includes external and internal components, for stimulating
tissue inside a patient;
FIG. 2 is a schematic diagram of an exemplary implanted medical
device with an external component containing a Class-E RF
amplifier;
FIG. 3 is a detailed schematic diagram of the Class-E RF amplifier;
and
FIG. 4 depicts waveforms for signals in the Class-E RF
amplifier.
DETAILED DESCRIPTION OF THE INVENTION
With initial reference to FIG. 1, a wireless transvascular platform
10 for tissue stimulation includes an extracorporeal power source
14 and a medical device 12 implanted inside the body 11 of an
animal. The extracorporeal power source 14 includes a battery that
powers a transmitter that sends a first radio frequency (RF) signal
26 to the medical device 12. The medical device 12 derives
electrical power from the energy of the first radio frequency
signal 26 uses that power to energize and electronic circuit 30
mounted on an electronic carrier 31. The first radio frequency
signal 26 also carries commands to configure the operation of the
medical device.
A second RF signal 28 enables the medical device 12 to transmit
operational data back to the extracorporeal power source 14. Such
data may include physiological conditions of the animal, status of
the medical device and trending logs, for example, that have been
collected by the implanted electronic circuit 30 and sent via the
second radio frequency signal 28. This data is provided transmitted
by the extracorporeal power source 14 monitoring equipment so that
medical personnel can review the data or be alerted when a
particular condition exists.
The implanted medical device 12 includes the electronic circuit 30
mentioned above which has an RF transceiver and a tissue
stimulation circuit, similar to that used in conventional
pacemakers and defibrillators. That electronic circuit 30 is
located in a large blood vessel 32, such as the inferior vena cava
(IVC), for example. One or more, electrical leads 33 and 34 extend
from the electronic circuit 30 through the animal's blood
vasculature to locations in the heart 36 where pacing and sensing
are desired. Each lead has an electrical conductor enclosed in an
electrically insulating outer layer. The electrical leads 33 and 34
terminate at electrode assemblies 38 at those locations.
With reference to FIG. 2, the internal components comprise an
implanted medical device 12 includes a stimulation circuit 132
having a first receive antenna 152 within the antenna assembly 124
in which the antenna is tuned to pick-up a first RF signal 26 at a
first radio frequency F1. The first receive antenna 152 is coupled
to a data detector 156 that recovers data and commands carried by
the first RF signal 26. That data specifies operational parameters
of the medical device 12, such as the duration that a stimulation
pulse is applied to the electrodes 120 and 121. The recovered data
is sent to a control circuit 155 for that medical device, which
stores the operational parameters for use in controlling operation
of a pacing signal generator 158 that applies tissue stimulating
voltage pulses across the electrodes 120 and 121.
The control circuit 155 also is connected to a pair of sensor
electrodes 157 that detect electrical activity of the heart and
provide conventional electrocardiogram signals which are utilized
to determine when cardiac pacing should occur. Additional sensors
for other physiological characteristics, such as temperature, blood
pressure or blood flow, may be provided and connected to the
control circuit 155. The control circuit stores a histogram of
pacing, data related to usage of the medical device, and other
information which can be communicated to the extracorporeal power
source 14 or another form of a data gathering device that is
external to the patient.
The first receive antenna 152 also is connected to a rectifier 150
that extracts energy from the received first RF signal. That energy
is used to charge a storage capacitor 154 that supplies electrical
power to the components of the implanted medical device 12.
Specifically, the radio frequency, first RF signal 26 is rectified
to produce a DC voltage (VDC) that is applied across the storage
capacitor 154.
The DC voltage produced by the rectifier 150 also is applied to a
feedback signal generator 160 comprising a voltage detector 162 and
a voltage controlled, first radio frequency oscillator 164. The
voltage detector 162 senses and compares the DC voltage to a
nominal voltage level desired for powering the medical device 12.
The result of that comparison is a control voltage that indicates
the relationship of the actual DC voltage derived from the received
first RF signal 26 and the nominal voltage level. The control
voltage is fed to the control input of the voltage controlled,
first radio frequency oscillator 164 which produces an output
signal at a radio frequency that varies as a function of the
control voltage. For example, the first radio frequency oscillator
164 has a center, or second frequency F2 from which the actual
output frequency varies in proportion to the polarity and magnitude
of the control signal and thus deviation of the actual DC voltage
from the nominal voltage. For example, the first radio frequency
oscillator 164 has a first frequency of 100 MHz and varies 100 kHz
per volt of the control voltage with the polarity of the control
voltage determining whether the oscillator frequency decreases or
increases from the second frequency F2. For this exemplary
oscillator, if the nominal voltage level is five volts and the
output of the rectifier 150 is four volts, or one volt less than
nominal, the output of the voltage controlled, first radio
frequency oscillator 164 is 99.900 MHz (100 MHz-100 kHz). That
output is applied to via a first data modulator 165 to a first
transmit antenna 166 of the implanted medical device 12, which
thereby emits a second RF signal 28. Data regarding physiological
conditions of the animal and the status of the medical device 12
are sent from the control circuit 155 to the first data modulator
165 which amplitude modulates the second RF signal 28 with that
data.
As noted previously, the electrical energy for powering the medical
device 12 is derived from the first RF signal sent by the
extracorporeal power source 14. The extracorporeal power source 14
uses power from a rechargeable battery 170 to periodically transmit
pulses of the first RF signal 26. The first RF signal 26 is pulse
width modulated to vary the magnitude of energy received by the
implanted medical device 12. The pulse width modulation is
manipulated to control the amount of energy the medical device
receives to ensure that it is sufficiently powered without wasting
energy from the battery 170 in the extracorporeal power source 14.
Alternatively, the first RF signal 26 can also be modulated by
amplitude modulation to vary the magnitude of energy received by
the implanted medical device 12.
To control the energy of the first RF signal 26, the extracorporeal
power source 14 contains a second receive antenna 174 that picks up
the second RF signal 28 from the implanted medical device 12.
Amplitude modulated data is extracted from the second RF signal 28
by a data receiver 116 and sent to the controller 106. Because the
second RF signal 28 also indicates the level of energy received by
medical device 12, this enables extracorporeal power source 14 to
determine whether medical device should receive more or less
energy. The second RF signal 28 is sent from the second receive
antenna 174 to a feedback controller 175 which comprises a
frequency shift detector 176 and a proportional-integral (PI)
controller 180. The second RF signal 28 is applied to the frequency
shift detector 176 which also receives a reference signal at the
second frequency F2 from a second radio frequency oscillator 178.
The frequency shift detector 176 compares the frequency of the
received second RF signal 28 to the second frequency F2 and
produces a deviation signal AF indicating a direction and an
amount, if any, that the frequency of the second RF signal has been
shifted from the second frequency F2. As described previously, the
voltage controlled, first radio frequency oscillator 164, in the
medical device 12, shifts the frequency of the second RF signal 28
by an amount that indicates the voltage from rectifier 150 and thus
the level of energy derived from the first RF signal 26 for
powering the implanted medical device 12.
The deviation signal .DELTA.F is applied to the input of the
proportional-integral controller 180 which applies a transfer
function given by the expression GAIN/(1+s.sub.i.tau.), where the
GAIN is a time independent constant gain factor of the feedback
loop, is a time coefficient in the LaPlace domain, and s.sub.i is
the LaPlace term containing the external frequency applied to the
system. The output of the proportional-integral controller 180 on
line 181 is an error signal indicating an amount that the voltage
(VDC) derived by the implanted medical device 12 from the first RF
signal 26 deviates from the nominal voltage level. That error
signal corresponds to an arithmetic difference between a setpoint
frequency and the product of a time independent constant gain
factor, and the time integral of the deviation signal.
The error signal is sent to the control input of a pulse width
modulator (PWM) 182 which forms an amplitude modulator within a
power transmitter 173 and produces at output signal that is on-off
modulated as directed by the error input. The output from the pulse
width modulator 182 is fed to a second data modulator 184 which
modulates the signal with data from the controller 106 for the
medical device 12. The second data modulator 184 feeds the RF
signal to a Class-E type RF power amplifier 186 from which the
signal is applied to a second transmit antenna 188.
In addition to transmitting electrical energy to the implanted
medical device 12, the extracorporeal power source 14 transmits
operational parameters which configure the functionality of the
medical device. The implanted medical device 12 also sends
operational data to the extracorporeal power supply. A data input
device, such as a personal computer 100, enables a physician or
other medical personnel to specify operating parameters for the
implanted medical device 12. Such operating parameters may define
the duration of each stimulation pulse, an interval between atrial
and ventricular pacing, and thresholds for initiating pacing. The
data defining those operating parameters are transferred to the
extracorporeal power source 14 via a connector 102 connected to the
input of a serial data interface 104. The data received by the
serial data interface 104 can be applied to a microprocessor based
controller 106 or stored directly in a memory 108.
FIG. 3 illustrates a unique Class-E amplifier 300 that is employed
as the RF power amplifier 186. The modifications comprise an
overrated switch with low channel resistance and feedback
capacitance, a drive circuit closely integrated with the switch, a
mechanism to tune components by adjusting the drive frequency, and
an oscillator the duty cycle of which is controlled by non-linearly
manipulating a sinusoidal drive signal.
The Class-E amplifier 300 is operated by a voltage or current of
the output signal from the second data modulator 184, which is
passed through an input matching network 355 in which the mixed
modulator signal is AC coupled to a fraction of the sine wave
signal and the base line is shifted by a suitable design parameter.
The waveform of the drive signal at the output of the second data
modulator 184 is depicted in FIG. 4. The drive signal is formed by
pulses of the first radio frequency F1 that are present during the
on time of the amplifier 300 wherein the pulse duty cycle is
determined by the signal on line 181 from the proportional-integral
controller 180. The period that the amplifier is on is given by
Ts.sub.ON=.eta..sub.1Tf, where Tf is the total time of on and off
periods that form one signal cycle, and .eta..sub.1 is the ratio of
on time and the total time. Note that Tf=1/F1. These higher
frequency pulses provide finer control of the drive signal without
affecting the first radio frequency F1, as occurred with prior
methods. Note that this unique pulse design also makes the design
more robust and relatively immune to load variations. Thus it
allows tuning of components by slight adjustment of drive frequency
and control of the output power of the amplifier.
The Class-E RF power amplifier 300 has a supply input connected to
a source of a supply voltage V.sub.E and coupled to ground by an
input capacitor 310. A choke 320 couples the supply voltage V.sub.E
to the switch 325. The choke 320 maintains the current that flows
through the switch 325 during its on time, such that after the
switch opens, the current flow is distributed between a shunt
capacitor 330 and a resonant tank circuit 335, that includes the
second transmit antenna 188. The ratio of this distribution is a
function of the phase of the periodic cycle of the resonant tank
circuit 335 and of the timings of the switch 325. For maximum
efficiency, the switch 325 should close while the voltage across
the shunt capacitor 330 is substantially to zero.
The switch 325 is a low impedance device, preferably a MOSFET. It
is important to over specify the switch 325 by preferably an order
of magnitude or more. For example, if the maximum expected current
is one ampere, the switch should be rated to handle a transient
current of up to ten amperes. The switch element has a low channel
resistance and low feedback capacitance. The channel resistance
preferably should be such that the arithmetic product of channel
resistance and the peak current rating of the switch is less than
3% of the supply voltage V.sub.E to the amplifier 300. The feedback
capacitance preferably should be such that it is less than 10% of
the input circuit capacitance. The drive circuit is closely
integrated with the switch 325 wherein the circuit board layout is
chosen based on the selected component configuration, for example
by mounting the components as close together as possible. In
addition, the loop containing the peak current is spatially located
in close proximity to the switch 325.
The tank circuit 335 couples an amplifier output node 340, that is
located between the choke 320 and the switch 325, to ground. The
tank circuit 335 approximates the resonant waveform that is
measurable in an inductively coupled load, as is represented by the
"body tissue coupled load" 380. The majority of the coupling with
the body tissue is inductive L.sub.COUPLING and losses associated
with that coupling are represented by R.sub.LOAD.
To maintain the oscillatory condition, it is desirable to have
either predictable phase and gain parameters or control over these
parameters. When a load is presented, the drive is increased to
meet a predefined setpoint, or a variable setpoint, alternatively a
combination of these two methods. In one implementation, it is
sufficient to provide a start condition that initially closes the
switch 325 for a limited period of time, followed by providing
feedback such that the switch is turned off when sufficient current
is detected through the tank circuit.
In addition to the power level feedback provided by the implanted
medical device 12, it is also possible to provide further feedback
control by sampling the output power level at the second transmit
antenna 188. One technique for controlling the energy of the first
radio frequency signal 26 uses a lower frequency pulse width
modulation method. Here, the average output power is sampled and
the amplifier is pulse width modulated at a frequency that is one
or more orders of magnitude lower than the first radio frequency
F1. In one example, the PWM frequency could be 200 kHz for a 20 MHz
Class E amplifier.
In this feedback version, the drive circuit varies the on-time (or
duty cycle) of the switch 325 in response to the output of the
power transmitter 173 as measured by a pickup coil 370 coupled to
the second transmit antenna 188. The voltage induced across the
pickup coil 370 is rectified and filtered by an RC network 375 to
provide a feedback voltage that is translated pulse width modulator
182 to a duty cycle of the drive signal, wherein a greater feedback
voltage translates to a lower duty cycle, and a lesser feedback
voltage translates to a higher duty cycle. Thus the duty cycle is
proportional to the measurement from the pickup coil 370.
The feedback circuit measures the field level generated under load
and proportions the drive (on-duration of the amplifier switch 325)
accordingly to maintain the oscillatory condition. The feedback
circuit may not be self starting. However, it could be operated as
a modified self oscillating circuit, in which there is a first
radio frequency F1 operated at a minimum idle current. A unique
feature of the present invention is the use of a sinusoidal
envelope voltage that is non-linearly manipulated to derive the
rectangular pulses. This enables the number of components in the
Class-E amplifier to be reduced substantially.
For linear applications, the PWM frequency must be selected in
conformity with the maximum bandwidth and phase linearity desired
in the filtered output signal. For example, the maximum frequency
components must be at least one half of the PWM frequency, but may
need to be lower depending on the maximum allowed phase variance,
which is caused by the digitization process.
The foregoing description was primarily directed to a preferred
embodiment of the invention. Although some attention was given to
various alternatives within the scope of the invention, it is
anticipated that one skilled in the art will likely realize
additional alternatives that are now apparent from disclosure of
embodiments of the invention. For example, the present invention
was described in the context of a device for cardiac stimulation,
but can be employed with other types of implanted medical device
systems. Accordingly, the scope of the invention should be
determined from the following claims and not limited by the above
disclosure.
* * * * *